Apparatus for analysis of aerosols

ABSTRACT

There is provided apparatus for the measurement of particle mass concentration of an aerosol and which comprises at least one aerosol inlet and at least one electrostatic precipitator which directs an aerosol flow towards a mass sensor on which the particles are collected, the sensor being housed in a magazine including a plurality of other sensors, each of which may be substituted for the sensor, and a control. The control selectively connects one of the at least one aerosol inlet with one of the sensors.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. provisional patentapplication Ser. No. 60/519,699, filed on Nov. 13, 2003, the disclosureof which is hereby incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to apparatus used for the analysis of theparticles in an aerosol, and in particular measurement of particle massconcentration and size distribution of an aerosol.

Apparatus having piezoelectric crystal sensors have been used for anumber of years to measure particle concentrations of aerosols. Suchsensors are described in “The Performance of Piezo-Electric CrystalSensors Used to Determine Aerosol Mass Concentration” by Daly S P andLundgrun D A, American Industrial Hygiene Assoc. Journal, July, 1975,p518-532, Piezo-Electrostatic Aerosol Mass Concentration Monitor by OlinJ G, Sem G J & Christenson D L, Amer. Ind. Hyg. Assoc. J. 32:209 (1970)and U.S. Pat. No. 3,653,253. Such apparatus include a piezoelectriccrystal sensor which is exposed to a gas stream containing particlesthat deposit on the surface of the crystal. By measuring the frequencyshift of the crystal and by knowing the mass sensitivity for thecrystal, it is possible to determine the mass accumulated on the crystalsurface. If the volume of the gas flow is known, the mass concentrationcan be determined.

A problem with the apparatus is that the crystal sensor can onlyfunction properly up to a maximum mass loading and, when used forenvironmental monitoring of particles, the sensor often becomesoverloaded before the expiration of a standard test period; for example,a crystal sensor may overload in 1 or 2 days within a 14-day monitoringperiod for monitoring environmental particulates. Furthermore, theproblems associated with the overloading of crystal sensors reduces theusefulness of such sensors for testing vehicle emissions.

The present invention provides improved apparatus which is useful forcontinuous monitoring of particulate concentrations, and which is alsouseful for the analysis of collected particles.

SUMMARY OF INVENTION

According to an aspect of the present invention, there is providedapparatus for the measurement of particle mass concentration of anaerosol and which comprises at least one aerosol inlet and at least oneelectrostatic precipitator which directs an aerosol flow towards a masssensor on which the particles are collected, said sensor being housed ina magazine including a plurality of other sensors, each of which may besubstituted for said sensor, and a control. The control selectivelyconnects one of said at least one aerosol inlet with one of the sensors.

Each precipitator may be mounted in an aperture in a support, and themagazine may be movably mounted on the support so that sensors housed inthe magazine may each be brought separately into alignment with arespective precipitator. The precipitator inlet may be provided with aheater which is controllable and may be used to vary the temperature ofthe aerosol flowing past the sensor.

The sensors in the illustrative embodiment are piezoelectric quartzcrystals which may be in the form of discs.

The magazine may be in the form of a carousel with each sensor alignedwith a respective precipitator by indexing the carousel relative to thesupport. The carousel may be rotatably mounted in a cylindrical recessin the support and is indexed by a motor, preferably a stepped motor,operated by said control means.

The at least one aerosol inlet may be a plurality of aerosol inlets. Avalve assembly may be provided to selectively connect certain ones ofthe aerosol inlets with certain ones of the sensors. The valve assemblymay be a rotary valve assembly that is rotated with an actuator, such asa stepper motor.

The sensor may be provided with a heater which is controllable to varythe temperature of the sensor which may be preset to any temperature asis desired. This allows the sensor to be used to weigh collecteddeposits after they have been subject to heat treatment.

The apparatus may have a plurality of precipitators, preferably two,each being aligned with a respective sensor. This configuration allowsfor two operations to be performed simultaneously, for example, oneprecipitator/sensor combination may be measuring particle concentration,and the other combination may be used for thermo-gravimetric analysis.

The Inlets of the two precipitators may differ in aerosol sizeclassification performance, for example, in cross-section, or one may beprovided with a particle remover, for different uses of the apparatus.

Yet another aspect of the invention provides apparatus for themeasurement of particle mass concentration of an aerosol and whichcomprises at least one electrostatic precipitator which directs anaerosol flow onto a mass sensor on which the particles are collected,wherein the sensor is provided with a heater which is controllable tovary the temperature of the sensor which may be preset to anytemperature as is desired.

Also, according to the invention, there is provided apparatus for themeasurement of particle mass concentration of an aerosol, and whichcomprises at least one electrostatic precipitator which directs anaerosol flow onto a mass sensor on which the particles are collected,wherein each precipitator has an inlet provided with a heater which iscontrollable to vary the inlet temperature.

A further invention provides apparatus for the analysis of particles inan aerosol and which comprises a plurality of electrostaticprecipitators which each direct an aerosol flow onto a respective masssensor on which the particles are collected, wherein the respectiveinlets of said precipitators differ one from the other in aerosol sizeclassification properties.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by way of example and with reference tothe accompanying drawings in which:

FIG. 1 is a schematic cross section through an aerosol electrostaticprecipitator;

FIG. 2 is a schematic cross section through an apparatus for measuringthe particle mass of an aerosol, according to the present invention;

FIG. 3 is a top plan view of a quartz crystal magazine as used in theapparatus of FIG. 2;

FIG. 4 is a schematic drawing of the control system of the apparatusshown in FIG. 2;

FIG. 5 is the same view as FIG. 2 of an alternative embodiment;

FIG. 6 a is a top plan view of a rotating valve; and

FIG. 6 b is a bottom plan view of the rotating valve in FIG. 6 a.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, there is shown an aerosol electrostaticprecipitator 10 having a body 12 with a particle charging chamber 13therein. The chamber 13 may have a temperature-controlled sample inlettube 14 through which the aerosol is introduced into the chamber 13. Thetemperature controlled inlet 14 helps prevent cold aerosol from coolingthe surface of a particle collector and prevents water vapor from beingdeposited. The inlet tube 14 may be made from an electrically conductivematerial in order to reduce losses of particles passing through. Thetube dimensions are chosen to minimize particle losses and ensure thatthe aerosol may be heated to a required temperature.

A corona needle 16 is located within the chamber 13 and is connected toa high voltage power supply 58 (FIG. 4) via electrical conductors 15.The chamber body 12 is typically made from an electrically insulatingmaterial and its inner end portion 18 (that is inner with respect to theapparatus) is provided with a plurality of ring seals 17 to provide agas tight seal with a support 32 (FIG. 2). The corona needle 16 isisolated from the body 12 and is usually easily removable for cleaning.

The inner end portion 18 of the body 12 has a hollow central spigot 19which, in use, directs the charged aerosol flow onto a quartz crystal 33(FIG. 2), and is surrounded by an outer annular space 21 which isconflicted to a radial outlet port 22 through which the aerosol exitsthe body 12. The outlet port 22 has seals 17 on each side thereof. Otherconfigurations of an electrostatic precipitator may be used.

Now, with reference to FIG. 2, there is shown in schematic formapparatus 30 for measuring the particle mass of an aerosol. Theapparatus includes a housing 31 which encloses at least one aerosolelectrostatic precipitator 10 sealingly mounted on a support 32. Theprecipitator 10 is located in an aperture 39 in the support and iscoaxially aligned with a quartz crystal 33 mounted in a metal holder 34.In use, as particles pass the needle 16, they pick up charges (positiveor negative charging maybe used) from the intense ion field near theneedle tip and are driven by the electric field toward the collectionsurface, in this case a quartz crystal surface. Electrostatic force isan effective means for particle collection especially for smallsubmicrometer sized particles.

The o-rings 17 seal the precipitator 10 within the aperture 39 in thesupport 32 and against the crystal holder 34 and the gas flow exits theapparatus via outlet 22 and aligned exit passageway 35 in the support.In use, the exit passageway 35 is connected to a vacuum pump 56 (seeFIG. 4).

With reference now also to FIG. 3, the holder 34 with its respectivecrystal 33 is one of a plurality of, preferably 12, such crystals andholders mounted in a magazine, preferably a carousel 36. The holders 34are formed from an electrically conductive material and the crystals areremovable from the respective holders for example, for analysis, forstorage of the sample, cleaning, and the like. The quartz crystals 33are preferably in the form of a disc, which, in the illustrativeembodiment, are about 14 mm in diameter. The holders 34 areequiangularly spaced around the carousel 36 which is housed in acylindrical recess 37 in the support 32 and is sealingly rotatablewithin the recess 37. Seals 38 on the outer surface of the carouselprovide a gas tight seal with the radially inner surface of the recessand the carousel is rotated by a step motor 39 via a drive shaft 41. Anexternal handle 42 is provided to enable the carousel to be both easilyinserted into and removed from the support 32.

With reference now also to FIG. 4, a control system of apparatus 30includes an electronic control 43 relating to the crystal 33. In theillustrative embodiment, control 43 is located within the carousel 36.The controls 43 include an oscillator circuit 51 to drive and measurethe crystal frequencies, heater controls 52 for the crystal temperature,and sensors 53 for measurement of corona current on the crystal exposedto the aerosol. The controls 43 store calibration data for the crystalsand are connected to a main control 45 and transmit data to the maincontrol 45. The piezoelectric quartz crystal 33 is externally driven bythe oscillator circuit 51 attached to two metal plates, preferablyplatinum or gold, located on either side of the crystal. This imposes atime-dependent electric field across the plates causing the crystal tooscillate. The resonant frequency is typically between 1 to 10 MHz, butmay be outside of this range depending upon application.

The control system of apparatus 30 includes a main control 45. Control45 may be located within the housing and may be provided within its ownelectrical power source 46, such as a long life battery. The control 45may be accessed through an external computer (not shown), such as a PC.The inlet 14 for the precipitator is connected to external inlets 47.

The main control 45 for that apparatus is shown schematically in FIG. 4and includes a main microprocessor control 45 which is connected to thecarousel controls 43 as previously described, as well as to the othercontrols. For the sake of clarity, the lines linking the controls havebeen omitted. The aerosol flow through the precipitator 10 is monitoredby a flow measure 55 connected to control 45 which also controls thevacuum pump 56. The field strength of the corona at the needle 16 isvaried by control of the power supply 58 connected to conductors 15 andvaried to provide a constant corona current. The temperature of theaerosol inlet 14 is controlled by means of a heater 61 and its controls62. Rotation of the carousel 32 is controlled by a motor positioncontrol 63 which causes the step motor 39 to index the carousel todifferent positions to bring different crystals into alignment with theprecipitator.

The plurality of crystal collectors allows the frequency of the crystalsnot exposed to the aerosol to be determined enabling the mass loss to bedetermined post aerosol exposure. Since this can be carried out atcontrolled crystal temperatures, this also enables a thermo-gravimetricanalysis to be performed on particle deposits collected.

The apparatus may be provided with a plurality of precipitators 10, andpreferably two, as shown in FIG. 2. This allows for different inlets tobe selected for each precipitator. For example, different size inletswill give particle sizing information and, if one inlet is provided witha particle removal system, the apparatus will have a thermal gravimetricanalysis capability allowing particle levels to be monitored at onelocation whilst thermo-gravimetric analysis is carried out at thesecond.

A second configuration 130 of an apparatus for analyzing particles in anaerosol is shown FIG. 5. In apparatus 130, there is provided apparatusfor the measurement of particle mass concentration of an aerosol, butthe said mass sensor is selected by a rotating multi-valve 67 mountedabove the sensor magazine. This valve can be rotated using a steppermotor 65 or any other electromechanical means. Apparatus 130 may have aplurality of precipitators/inlet heaters, preferably eight, each beingselectively aligned with a respective sensor. This configuration allowsfor multiple operations to be performed simultaneously, for example, oneprecipitator/sensor combination may be used for measuring the particleconcentration, another may be used for thermo-gravimetric analysis and athird for a sample baseline determination (for example, humiditycompensation). A cassette of filters 66 used for gravimetric or otheradditional measurements may be provided, such a cassette is locatedabove the main body. These filters can be automatically selected tofacilitate a simultaneous collection of the sampled aerosol.

With reference now also to FIG. 5, the holder 34 with its respectivecrystal 33 is one of a plurality of, preferably eight, such crystals andthese holders are mounted in a magazine 64. An external handle 73 isprovided to enable the carousel to be both easily inserted into andremoved from the support 72. The sample is directed to the respectivecrystal 33 by a rotating sampling valve 67.

The apparatus may be provided with a rotating sampling valve 67 toautomatically switch the various sampling inlets 68, 69, 70, 71 to thecorresponding sensor. The rotating sampling valve is controlled using astepper motor 65 or other suitable electromechanical means. By rotatingthe valve, the sample inputs, which are illustrated at 68, 69, 70, 71,can be directed to any respective crystal 33 and a gravimetric filter66. The top filter block 80 can be removed to facilitate filterchanging.

The valve rotating valve 67 is sealed to the bottom mounting block 78and the top filter mounting block 77 using “o” rings 81 (FIG. 6). Thevalve rotating block 67 is also sealed to the valve outer block 79 by“o” rings 75.

With reference now also to FIG. 5, electronic controls 43 relating tothe sensor 33 may be located within the carousel 64. The controls 64include oscillator circuits 51 to drive and measure the crystalfrequencies, heater controls 52 for the crystal temperatures, andsensors 53 for the measurement of corona current on the crystal exposedto the aerosol.

The apparatus is controlled through a main control 45.

Humidity compensation of the sampling sensor 33 can be achieved bysimultaneously sampling a proportion of the filtered sample 74 to asecond sensor 33.

Filter collection 66 can be made in parallel with the sensor 33 massdetermination.

The control 45 may be programmed to automatically change the samplingcrystal according to different parameters, for example:

1) when the crystal reaches its maximum mass loading,

2) after a specified time lapse,

3) after sampling in a particular location,

4) after sampling at particular inlet temperatures,

5) on manual (or other) external trigger.

The automatic changing of the crystal minimizes the limitations of theprior art apparatus associated with overloading the crystal, particlecomposition, and chemical properties of the particles.

Independent temperature control of the inlet 14 and the quartz crystalenables an assessment of the volatile/non-volatile components of theparticles to be made. The control 45 also allows for the collection andstorage temperatures of the crystals to be pre-programmed.

The multi-holder arrangement is not confined to a carousel and holdersmay be provided in a magazine in a linear array either side by side orin a stack.

The magazine, including the carousel, may be provided with an in-situcrystal cleaning system.

Changes and modifications in the specifically described embodiments canbe carried out without departing from the principles of the inventionwhich is intended to be limited only by the scope of the appendedclaims, as interpreted according to the principles of patent lawincluding the doctrine of equivalents.

1. Apparatus for the analysis of particles in an aerosol, comprising: at least one aerosol inlet and at least one electrostatic precipitator which directs an aerosol flow onto a mass sensor on which the particles are collected, the sensor being housed in a magazine including a plurality of other sensors each of which may be substituted for said sensor, and a control, said control selectively connecting one of said at least one aerosol inlet with one of said sensors.
 2. Apparatus as claimed in claim 1 wherein each precipitator is mounted in an aperture in a support, and the magazine is movably mounted on the support so that sensors housed in the magazine may each be brought separately into alignment with the precipitator.
 3. Apparatus as claimed in claim 2 wherein the magazine is in the form of a carousel and each sensor is aligned with a respective precipitator by indexing the carousel relative to the support.
 4. Apparatus as claimed in claim 3 wherein the carousel, is rotatably mounted in a cylindrical recess in the support and is indexed by a motor operated by said control means.
 5. Apparatus as claimed in claim 1 wherein the sensor is provided with a heater which is controllable to vary the temperature of the sensor which may be preset to any temperature as is desired.
 6. Apparatus as claimed in claim 5 wherein sensor heater controls and mass monitoring controls are housed within the magazine.
 7. Apparatus as claimed in claim 1 wherein said at least one precipitator has an inlet provided with a heater which is controllable to vary the inlet temperature.
 8. Apparatus as claimed in claim 1 wherein the mass sensor is a piezoelectric quartz crystal.
 9. Apparatus as claimed in claim 1 wherein said at least one sensor comprises at least two precipitators each being aligned with a respective sensor.
 10. Apparatus as claimed in claim 9 wherein the respective inlets of said precipitators differ one from the other.
 11. Apparatus as claimed in claim 10 wherein the inlets have different aerosol size classification properties.
 12. Apparatus as claimed in claim 10 wherein one of said inlets is provided with a particle removal system.
 13. Apparatus as claimed in claim 1 wherein said at least one aerosol inlet comprises a plurality of aerosol inlets.
 14. Apparatus as claimed in claim 13 including a valve assembly, said valve assembly selectively connecting particular ones of said aerosol inlets with certain ones of the sensors.
 15. Apparatus as claimed in claim 14 wherein said valve assembly comprises a rotary valve assembly.
 16. Apparatus as claimed in claim 15 including an actuator for selectively rotating said rotary valve assembly.
 17. A method of analyzing particle mass concentration of an aerosol which includes the use of apparatus as claimed in claim
 1. 18. Apparatus for the analysis of particles in an aerosol and which comprises at least one electrostatic precipitator which directs an aerosol flow onto a mass sensor on which the particles are collected, the sensor being housed in a magazine including a plurality of other sensors each of which may be substituted for said sensor, and control means for automatically operating the magazine to effect a change of sensor. 